The paper by Benham gives a full description of the musculature of Limulus, and according to his arrangement the muscles are divided into two sets, longitudinal and dorso-ventral. Of the latter, each mesosomatic segment possesses a pair of dorso-ventral muscles, attached to the mid-ventral mesosomatic entochondrite, and to the tergal surface (Fig. [58], Dv.). These muscles are called by Benham the vertical mesosomatic muscles. I shall call them the somatic dorso-ventral muscles, in contradistinction to the dorso-ventral muscles of the branchial appendages. Of the latter, the two chief are the external branchial (Fig. [66], m3) and the posterior entapophysio-branchial (Fig. [66], m1); a third muscle is the anterior entapophysio-branchial (Fig. [66], m2). Of these muscles, the posterior entapophysio-branchial (m1) is closely attached along the branchial cartilaginous bar up to its round-headed termination on the anterior surface of the appendage. The anterior entapophysio-branchial muscle (m2) is attached to the branchial cartilage near the entapophysis.
In the case of the scorpion, as described by Miss Beck, the branchial appendage has become reduced to the branchiæ, and the intrinsic appendage-muscles have entirely disappeared, with the possible exception of the small post-stigmatic muscle; on the other hand, the dorso-ventral somatic muscles, which are clearly homologous with the corresponding muscles of Limulus, have remained, and become the essential respiratory muscles.
Of these two possible types of respiratory movement it is quite conceivable that in the water-breathing scorpions of olden times and in their allies, the dorso-ventral muscles of their branchial appendages may have continued their rôle of respiratory muscles, and so have given origin to the respiratory muscles of the ancestors of Ammocœtes.
The respiratory muscles of Ammocœtes are three in number, and have been described by Nestler and Miss Alcock as the adductor muscle, the striated constrictor muscle, and the tubular constrictor muscle (Fig. [65], m. add., m.c.s., and m.c.t.). Of these, the constrictor muscle (Fig. [71], m. con. str.) is in close contact with its cartilaginous bar, while the adductor (Fig. [71], m. add.) is attached to the cartilage only at its origin and insertion, and the tubular muscles (Fig. [71], m. con. tub.) have nothing whatever to do with the cartilage at all, being attached ventrally to the connective tissue in the neighbourhood of the ventral aorta (V.A.), and dorsally to the mid-line between the dorsal aorta (D.A.) and the notochord.
The close relationship of the constrictor muscle to the cartilaginous branchial bar does not favour the surmise that this muscle is homologous with the dorso-ventral somatic muscle of the scorpion. It is, however, directly in accordance with the view that this muscle is homologous with one of the dorso-ventral appendage-muscles, such as the posterior entapophysio-branchial muscle (m1, Fig. [66]) of the Limulus appendage, especially when the homology of the Ammocœtes branchial cartilage with the Limulus branchial cartilage is borne in mind. I am, therefore, inclined to look upon the constrictor and adductor muscles of the Ammocœtes branchial segment as more likely to have been derived from dorso-ventral muscles which belonged originally to a branchial appendage, such as we see in Limulus, than from dorso-ventral somatic muscles, such as the vertical mesosomatic muscles which are found both in Limulus and scorpion. In other words, I am inclined to hold the view that the somatic dorso-ventral muscles have disappeared in this region in Ammocœtes, while dorso-ventral appendage-muscles have been retained, i.e. the exact reverse to what has taken place in the air-breathing scorpion.
I am especially inclined to this view because of the manner in which it fits in with and explains van Wijhe's results. Ever since Schneider divided the striated muscles of vertebrates into parietal and visceral, such a division has received general acceptance and, as far as the head-region is concerned, has received an explanation in van Wijhe's work; for Schneider's grouping corresponds exactly to the two segmentations of the head-mesoblast, discovered by van Wijhe, i.e. to the somatic and splanchnic striated muscles according to my nomenclature. Of these two groups the splanchnic or visceral striated musculature, innervated by the Vth, VIIth, IXth, and Xth nerves, which ought on this theory to be derived from the musculature of the corresponding appendages, is, speaking generally, dorso-ventral in direction in Ammocœtes and of the same character throughout; the somatic musculature, on the other hand, is clearly divisible, in the head region, into two sets—a spinal and a cranial set. The somatic muscles innervated by the spinal set of nerves, including in this term the spino-occipital or so-called hypoglossal nerves, are in Ammocœtes most sharply defined from all the other muscles of the body. They form the great dorsal and ventral longitudinal body-muscles, which extend dorsally as far forward as the nose and are developed embryologically quite distinctly from the others, being formed as muscle-plates (Kästchen). On the other hand, the cranial somatic muscles are the eye-muscles, the formation of which resembles that of the visceral muscles, and not of the spinal somatic. Their direction is not longitudinal, but dorso-ventral; they cannot, in my opinion, be referred to the somatic trunk-muscles, and must, therefore, form a separate group to themselves. Thus the striated musculature of the Ammocœtes must be divided into (1) the visceral muscles; (2) the longitudinal somatic muscles; and (3) the dorso-ventral somatic muscles. Of these the 1st, on the view just stated, represent the original appendage-muscles; the 2nd belong to the spinal region, and will be considered with that region; the 3rd represent the original segmental dorso-ventral somatic muscles, which are so conspicuous in the musculature of the Limulus and the scorpion group.
The discussion of this last statement will be given when I come to deal with the prosomatic segments of Ammocœtes. I wish, here, simply to point out that van Wijhe has shown that the eye-muscles develop from his 1st, 2nd, and 3rd dorsal mesoblastic segments, and therefore represent the somatic muscles belonging to those segments, while no development of any corresponding muscles takes place in the 4th, 5th, and 6th segments; so that if the eye-muscles represent a group of dorso-ventral somatic muscles, such muscles have been lost in the 4th, 5th, and 6th segments. The latter segments are, however, the glossopharyngeal and vagus segments, the branchial musculature of which is derived from the ventral segments of the mesoderm. In other words, van Wijhe's observations mean that the dorso-ventral somatic musculature has been lost in the branchial or mesosomatic region, while the dorso-ventral appendage musculature has been retained, and that, therefore, the mode of respiration in Ammocœtes more closely resembles that of Limulus than of Scorpio.
In addition to these branchial muscles, another and very striking set of muscles is found in the respiratory region of Ammocœtes—the so-called tubular muscles. These muscles are of great interest, but as they are especially connected with the VIIth nerve, their consideration is best postponed to the chapter dealing with that nerve.
Also, in connection with the vagus group of nerves, special sense-organs are found in the skin covering this mesosomatic region, the so-called epithelial pit-organs (Ep. pit., Fig. [71]). They, too, are of great interest, but their consideration may also better be deferred to the chapter dealing with those special sense-systems known as the lateral line and auditory systems.
Comparison of the Branchial Circulation in Ammocœtes and Limulus.